1. Field of the Invention
The present invention relates to image forming apparatus such as copying machines and printers, and more particularly to image forming apparatus having means for detecting whether the copy paper to be used is transparent paper for overhead projectors (hereinafter referred to as "OHP paper") or plain paper.
2. Description of the Prior Art
When images are to be formed on OHP paper by electrophotographic image forming apparatus, the transfer output and fixing speed for forming images on plain paper must be altered in view of the difference between the OHP paper and the plain paper in properties.
Accordingly, image forming apparatus are already available wherein OHP paper is supplied to the main body thereof only through a manual feed portion and which are adapted to alter the transfer output, etc. when an OHP paper sensor provided for the manual feed portion detects OHP paper by checking whether light passes through the copy paper manually fed.
However, the image forming apparatus has the drawback that the operator must feed OHP paper one sheet after another when many copies of OHP paper are to be made since the OHP paper is not feedable to the apparatus body from a paper cassette.
The above drawback can be overcome by providing another OHP paper sensor, like the one mentioned, at a suitable portion of the path of transport of the copy paper to be fed from the paper cassette so that the OHP paper can be fed not only from the manual feed portion but also from the cassette.
Nevertheless, another problem will then be encountered. Since OHP paper sensors of the transmission type are expensive, use of the two OHP paper sensors makes the image forming apparatus more costly.